CHARACTERISTICS OF DATA COLLECTED BY
LTPP'S PROFILERS

K.J. Law Engineers DNC 690
Profiler

The DNC 690 profiler collected profile data at 25.4-mm (1-inch)
intervals, and then applied a 304.8-mm (12-inch) moving average
onto the data and recorded the data at 152.4-mm (6-inch) intervals.
The data collected by this profiler at LTPP sections and the IRI
values computed from the profile data are available in the LTPP
database. Figure 19 shows a PSD plot of the data collected by this
profiler. The PSD plot shows a sharp drop after a wave number of 1
cycle/m (0.3 cycle/ft), which corresponds to a wavelength of 1 m (3
ft). This sharp drop in the PSD plot is an indication that a moving
average has been applied to the profile data. The application of
the moving average onto the profile data attenuates wavelengths
less than 1 m (3 ft).

K.J. Law Engineers T-6600
Profiler

The T-6600 profiler recorded profile data at 25-mm (1-inch)
intervals. In the LTPP program, these data are processed using the
ProQual software, which applies a 300-mm (11.8-inch) moving average
onto the 25-mm (1-inch) interval profile data, and then extracts
profile data points at 150-mm intervals. ProQual computes the IRI
using these averaged data. The IRI values and the averaged 150-mm
(5.9-inch) interval profile data for LTPP sections are available in
the LTPP database.

Figure 20 shows a PSD plot of the 25-mm (1-inch) data collected
by the T-6600 profiler and the PSD plot of the same data after it
has been processed using ProQual. Figure 20 shows that there is a
significant difference in the profile content between the two
profilers for wave numbers greater than 1 cycle/m (0.3 cycle/ft),
which corresponds to wavelengths less than 1 m (3 ft). The sharp
dropoff seen in the PSD plot for 150-mm (5.9-inch) data for wave
numbers greater than 1 cycle/m (0.3 cycle/ft) occurs because the
moving average attenuates wavelengths less than 1 m (3 ft) in the
profile.

International Cybernetics
Corporation Profiler

The ICC profilers do not record profile data, but rather record
the data collected by the height sensors, accelerometers, and the
DMI. These data can be used to generate profiles with a 25-mm
(1-inch) sampling interval. In the LTPP program, these 25-mm
(1-inch) data are processed using the ProQual software, which uses
the same procedure as described for the T-6600 profiler. As in the
case of the T-6600 profiler, IRI is computed using the averaged
data that are at 150-mm (5.9-inch) intervals. The computed IRI
values and the averaged profile data for LTPP sections are
available in the LTPP database. Figure 21 shows a PSD plot of the
25-mm (1-inch) data collected by the ICC profiler and the PSD plot
of the same data after it had been processed using ProQual.

The trend between the 25-mm (1-inch) data and the 150-mm
(5.9-inch) data seen in this figure is similar to the trend that
was observed for the T-6600 profiler, where the application of the
moving average caused profile features that have a wave number
greater than 1 cycle/m (0.3 cycle/ft) to become attenuated.

COMPARISON OF K.J. LAW ENGINEERS DNC 690 AND T-6600
PROFILERS

Comparison of Profile Data

The DNC 690 profiler recorded profile data at 152.4-mm (6-inch)
intervals, while the T-6600 profiler recorded profile data at 25-mm
(1-inch) intervals. The DNC 690 and T-6600 profilers applied a 91-m
(300-ft) and 100-m (328-ft) upper-wavelength cutoff filter to the
profile data, respectively.

1 cycle/m = 0.3 cycle/ft
1 m/cycle = 3 ft/cycle

Figure 21. PSD plot of data collected by
the ICC profiler.

Figure 22 shows overlaid profile plots of data collected at the
same site by the DNC 690 and T-6600 profilers. These data are from
the smooth AC section that was used by the North Central region for
the 1996 profiler verification test.(24) Figure 23 shows
a similar plot at the rough AC section used by the North Central
region for the same study.

2.54 mm = 1 inch
1 m = 3.28 ft

Figure 22. Data collected by the North
Central K.J. Law Engineers DNC 690 and T-6600 profilers at the
smooth AC site during the 1996 verification test.

2.54 mm = 1 inch
1 m = 3.28 ft

Figure 23. Data collected by the North
Central K.J. Law Engineers DNC 690 and T-6600 profilers at the
rough AC site during the 1996 verification test.

The data collected by the two profilers overlay extremely well
at the smooth AC section, while at the rough AC section, the
agreement is less when compared to the agreement seen at the smooth
AC section. At the rough AC section, although there is a slight
shift between the two profiles, an evaluation of the profile data
using filtering techniques shows that similar profile features are
captured by both profilers.

The overall shape of a profile plot primarily depends on the
long-wavelength content in the profile. There is a slight
difference in the long-wavelength cutoff limit used by the two
profilers, which can cause some differences to occur in the profile
plots. The long-wavelength content at the rough AC site is higher
than that at the smooth AC site and, thus, differences among the
profiles are seen more clearly at the rough AC site. These
observations, as well as a comparison of other data collected
during the 1996 verification test, indicate that the same
upper-wavelength cutoff filtering technique appears to have been
used with the DNC 690 and T-6600 profilers.

Figure 24 shows PSD plots of left-wheelpath data collected by
the DNC 690 profiler, which has a sampling interval of 152.4 mm (6
inches), and the T-6600 profiler, which has a sampling interval of
25 mm (1 inch), at the smooth AC site in the North Central region
during the 1996 verification test. The PSD plots show good
agreement, except for wave numbers greater than 1 cycle/m (0.3
cycle/ft), which corresponds to wavelengths less than 1 m (3 ft).
In this waveband range, the profile content in the DNC 690 profiler
is attenuated when compared to the T-6600 profiler. This
attenuation is caused by the moving average filter that is applied
to the DNC 690 profiler data before saving the data.

Figure 25 shows the PSD plot of the two data sets whose PSD
plots are shown in figure 24, except that the data shown for the
T-6600 profiler are the data that were obtained after the 25-mm
(1-inch) data were processed using ProQual. The application of the
300-mm (11.8-inch) moving average onto the 25-mm (1-inch) T-6600
profiler data attenuates wavelengths less than 1 m (3 ft), which
corresponds to wave numbers greater than 1 cycle/m (0.3 cycle/ft).
The two PSD plots shown in figure 25 indicate good agreement. A
review of similar plots for other data collected during the 1996
verification test showed similar trends. These results confirm that
the DNC 690 profiler applied a 304.8-mm (12-inch) moving average
onto the data before saving the data. Since the PSD plots for the
two profilers agree well through a range of 0.025 to 1 cycle/m
(0.008 to 0.3 cycle/ft), which corresponds to wavelengths between 1
and 40 m (3 and 130 ft), the IRI values of the two profilers are
expected to agree closely.

The PSD plots also indicate that the spectral content of the
data collected at the same site by the two profilers was similar.
This indicates that the profile features that are recorded by the
two profilers are similar. (Note: These observations are only valid
when the DNC 690 profiler data are compared to ProQual-processed
T-6600 profiler data. If 25-mm (1-inch) T-6600 profiler data are
compared to DNC 690 profiler data, differences among the profile
data will be seen for wave numbers greater than 1 cycle/m (0.3
cycle/ft).) The PSD plots also indicated good agreement between the
two profilers for low wave numbers (long wavelengths), which is an
indication that the filtering technique used by the two profilers
for the upper-wavelength cutoff is similar. The slight differences
seen in the PSD plots for the low wave numbers (long wavelengths)
are probably related to the different upper-wavelength cutoff
values that were used in the two profilers, which are 91 m (300 ft)
for the DNC 690 profiler and 100 m (328 ft) for the T-6600
profiler.

Comparison of IRI Values

When comparison testing between the two profilers is performed,
the profiler driver should do the following two tasks
accurately:

Align the profiler along the wheelpaths.

Maintain a consistent path within the test section.

If the profiler driver does not correctly align the profiler
along the wheelpaths, the longitudinal path followed by the sensors
of the different profilers will be different. This can cause IRI
obtained from the different profilers to vary. After aligning the
profiler along the wheelpaths, the driver should also follow a
consistent path within the test section without lateral wander. If
the is variability in the path that is followed within the section,
it can result in differences in IRI when the two profilers are
being compared.

The ability of a driver to correctly align the profiler along
the wheelpaths and to follow a consistent path within the section
can vary among drivers. Drivers who are more experienced in
profiling can probably do these two tasks much better than a driver
who is inexperienced in profiling. A driver who is experienced in
profiling probably will also be able to follow a more consistent
path when obtaining repeat measurements at a site than a person who
does not have much experience in operating the profiler.

The ability of a driver to correctly align the profiler along
the wheelpaths and to follow a consistent path within the section
can vary among drivers. Drivers who are more experienced in
profiling can probably do these two tasks much better than a driver
who is inexperienced in profiling. A driver who is experienced in
profiling probably will also be able to follow a more consistent
path when obtaining repeat measurements at a site than a person who
does not have much experience in operating the profiler.

Not following the correct wheelpath, or variability within the
section during profiling, will usually have a greater impact on the
IRI for sections that have distresses. This is because, in such
sections, lateral variations can cause certain pavement features
either to be included or missed in the profile, thus affecting the
IRI. The effect of lateral wander is an issue that can sometimes
complicate the analysis when the IRI from different profilers are
compared.

A study performed for an NCHRP project found that variations in
the longitudinal path that is followed during profiling could have
a significant effect on IRI.(1) In this study, the
effect of lateral variations in profiling was studied at seven test
sections. The percent change in IRI that was obtained for each
wheelpath at the test sections for a 0.3-m (1-ft) lateral shift in
the longitudinal path to the left and to the right is shown in
table 3. As shown in this table, the percent change in IRI will
vary for different pavements. Some pavements showed extremely large
variations in IRI for a 0.3-m (1-ft) shift from the wheelpath.
Generally, the percent change in IRI that was observed along the
left wheelpath was less than that obtained for the right
wheelpath.

Table 3. Changes in IRI caused by lateral variations in
the longitudinal path.

Section

IRI (m/km)

Percent Change in IRI

Wheelpath

Shifted 0.3 m Right

Shifted 0.3 m Left

Left

Right

Wheelpath

Wheelpath

Left

Right

Left

Right

New AC

0.85

0.98

-4

6

-4

-7

AC with Transverse Cracks

1.20

1.22

-3

34

8

2

Old AC

1.85

2.63

-7

0

-17

-22

1-year-old PCC

0.84

1.41

1

13

-4

-21

3-year-old PCC

0.59

0.58

-5

14

15

7

6-year-old PCC

1.41

1.75

4

13

-4

-3

Severely Faulted PCC

3.67

3.83

-1

1

8

-2

1 m = 3.28 ft
1 m/km = 5.28 ft/mi

After each regional contractor accepted delivery of the T-6600
profiler in 1996, they performed a comparison of this profiler and
the DNC 690 profiler. Four test sections were used in each region
to perform this comparison. (See references 24, 25, 26, and 27.)
The results obtained from this test are described in appendix B. In
the reports prepared by the regional contractors, the average IRI
value for a wheelpath from the multiple runs that were performed at
a site was computed using different procedures. In this research
project, a consistent method was used on data obtained from all
four regions to compare the IRI values between the DNC 690 and
T-6600 profilers. The IRI values obtained for sequence 2 testing
were used in this analysis (except at site 1 in the Western region
for the DNC 690 profiler, where sequence 1 values were used because
sequence 2 data had saturation spikes). The average IRI for each
wheelpath at each test section was computed using the IRI for the
five runs that had the least standard deviations in IRI for the
mean IRI (i.e., the average IRI for the left and right wheelpaths).
The computed average IRI values for both profilers for all regions
are shown in table 4. When all 4 regions were considered, there
were a total of 16 test sites (32 wheelpaths) where IRI comparisons
between the 2 profilers could be made.

Figure 26 shows the IRI relationship between the two profilers,
where data obtained for 32 wheelpaths are shown. There is very good
agreement in the IRI values obtained by the two profilers, except
for one case along the left wheelpath. This data point corresponds
to the left wheelpath of site 2 in the Western region. An
evaluation of the data indicated that the data collected by the DNC
690 profiler had a significant number of spikes that were caused by
sunlight being picked up by the sensor, which resulted in a high
IRI value. The correlation coefficient for the data shown in figure
26 is 0.98.

Table 4. IRI values from the 1996 verification
test.

Region

Site

Description

Average IRI (m/km)

Left Wheelpath

Right Wheelpath

DNC 690

T-6600

DNC 690

T-6600

North Atlantic

1

Smooth AC

0.86

0.74

0.86

0.88

North Atlantic

2

Rough AC

2.27

2.11

1.93

1.67

North Atlantic

3

Smooth PCC

1.22

1.18

1.31

1.34

North Atlantic

4

Rough PCC

1.87

1.90

2.15

2.24

North Central

1

Smooth AC

1.01

1.05

1.06

1.05

North Central

2

Rough AC

3.96

4.00

4.92

4.79

North Central

3

Smooth PCC

1.10

1.10

1.07

1.08

North Central

4

Rough PCC

2.69

2.67

2.99

2.95

Southern

1

Smooth AC

0.74

0.71

0.74

0.67

Southern

2

Rough AC

1.63

1.88

1.91

1.80

Southern

3

Smooth PCC

1.77

1.81

1.75

1.70

Southern

4

Rough PCC

2.17

2.01

2.50

2.38

Western

1

Smooth AC

0.77

0.78

0.90

0.92

Western

2

Rough AC

3.74

2.62

2.61

2.57

Western

3

Smooth PCC

1.04

0.99

0.93

0.89

Western

4

Rough PCC

2.31

2.35

2.40

2.39

1 m/km = 5.28 ft/mi

1 m/km = 5.28 ft/mi

Figure 26. Relationship between IRI from
the K.J. Law Engineers DNC 690 and T-6600 profilers.

The difference in IRI between the DNC 690 and T-6600 profilers
(DNC 690 IRI — T-6600 IRI) was computed along each wheelpath for
all of the test sections. These data are shown in figure 27 as a
function of the IRI for the wheelpath, where the IRI for the
wheelpath was computed by averaging the IRI obtained for that
wheelpath by the DNC 690 and T-6600 profilers. Of the 32 available
cases, the difference in IRI was within ±0.10 m/km (±6 inches/mi)
for 23 cases. For six cases, the difference was between 0.10 and
0.20 m/km (6 and 13 inches/mi). There was one case where the
difference was between -0.20 and -0.30 m/km (-13 and -19
inches/mi), one case where the difference was between 0.20 and 0.30
m/km (13 and 19 inches/mi), and one case where the difference was
greater than 1.1 m/km (70 inches/mi) (this data point is not shown
in figure 27).

1 m/km = 5.28 ft/mi

Figure 27. Differences in IRI between the
K.J. Law Engineers DNC 690 and T-6600 profilers: All
regions.

An investigation was performed separately for each region to
identify the cause of the difference in IRI between the two
profilers for cases where the difference in IRI was outside ±0.10
m/km (±6 inches/mi). In each region, the difference in IRI between
the DNC 690 and T-6600 profilers (DNC 690 IRI — T-6600 IRI) for
each wheelpath was plotted as a function of the IRI for the
wheelpath, with the IRI for the wheelpath computed by averaging IRI
obtained by the two profilers for that wheelpath.

North Atlantic Region

Figure 28 shows the difference in IRI from the DNC 690 and
T-6600 profilers at the test sections tested by the North Atlantic
profilers as a function of the IRI for the wheelpath.

A difference in IRI between the DNC 690 and T-6600 profilers
(DNC 690 — IRI — T-6600 IRI) that was outside ±0.10 m/km (±6
inches/mi) was observed for the following three cases: (1) left
wheelpath of site 1 (a difference of 0.12 m/km (8 inches/mi)), (2)
left wheelpath of site 2 (a difference of 0.16 m/km (10
inches/mi)), and (3) right wheelpath of site 2 (a difference of
0.26 m/km (16 inches/mi)).

Site 1: The left-wheelpath IRI from the DNC 690
profiler was 0.12 m/km (8 inches/mi) higher than IRI obtained by
the T-6600 profiler. The DNC 690 profiler conducted six runs on
this section. IRI along the left wheelpath for these six runs
ranged from 0.83 to 0.87 m/km (53 to 55 inches/mi). The T-6600
profiler conducted nine runs at this site, and the left-wheelpath
IRI for eight runs ranged from 0.72 to 0.77 m/km (46 to 49
inches/mi); however, there was one run that had an IRI of 0.85 m/km
(54 inches/mi). The roughness profile of this run overlaid
extremely well with the roughness profiles obtained by the DNC 690
profiler. This indicates that the probable cause of the difference
in IRI between the two profilers was a difference in the paths that
were followed during profiling.

1 m/km = 5.28 ft/mi

Figure 28. Differences in IRI between the
K.J. Law Engineers DNC 690 and T-6600 profilers: North Atlantic
region.

Site 2: IRI from the DNC 690 profiler was
higher than IRI from the T-6600 profiler by 0.16 m/km (10
inches/mi) and 0.26 m/km (16 inches/mi) for the left and right
wheelpaths, respectively. This analysis used IRI values obtained
during sequence 2 testing. The average IRI from the T-6600 profiler
from sequence 1 testing at this site, along the left and right
wheelpaths, were 2.21 m/km (140 inches/mi) and 1.89 m/km (120
inches/mi), respectively. These values compare extremely well with
the IRI values obtained for the left and right wheelpaths by the
DNC 690 profiler (2.27 m/km (144 inches/mi) and 1.93 m/km (122
inches/mi), respectively). This section had significant transverse
and longitudinal cracking along both wheelpaths throughout the test
section. Thus, the differences in IRI that were observed between
the two profilers for sequence 2 testing are attributed to
variations in the wheelpaths followed by the two profilers.

North Central Region

Figure 29 shows the differences in IRI from the DNC 690 and
T-6600 profilers at the test sections in the North Central region
as a function of the IRI for the wheelpath.

A difference in IRI between the DNC 690 and T-6600 profilers
that was outside ±0.10 m/km (±6 inches/mi) was observed only along
the right wheelpath at site 2, where the IRI from the DNC 690
profiler was higher than that from the T-6000 profiler by 0.13 m/km
(8 inches/mi). The right wheelpath at this site is extremely rough.
The right-wheelpath IRI for the five selected runs from the DNC 690
profiler ranged from 4.77 to 4.96 m/km (302 to 314 inches/mi),
while the IRI for the five selected runs from the T-6600 profiler
ranged from 4.74 to 4.81 m/km (301 to 305 inches/mi). These ranges
for the two profilers have some overlap. The difference in IRI
between the two profilers at this site is attributed to variations
in the wheelpaths.

1 m/km = 5.28 ft/mi

Figure 29. Differences in IRI between the
K.J. Law Engineers DNC 690 and T-6600 profilers: North Central
region.

Southern Region

Figure 30 shows the difference in IRI from the DNC 690 and
T-6600 profilers at the test sections tested by the Southern
profilers as a function of the IRI for the wheelpath.

Differences in IRI between the DNC 690 and T-6600 profilers (DNC
690 — IRI — T-6600 IRI) that were outside ±0.10 m/km (±6 inches/mi)
were observed for the following four cases: (1) left wheelpath of
site 2 (a difference of -0.25 m/km (-16 inches/mi)), (2) right
wheelpath of site 2 (a difference of 0.11 m/km (10 inches/mi)), (3)
left wheelpath of site 4 (a difference of 0.16 m/km (10
inches/mi)), and (4) right wheelpath of site 4 (a difference of
0.12 m/km (7 inches/mi)).

Site 2: At this site, IRI from the DNC 690
profiler was lower than that from the T-6600 profiler along the
left wheelpath by 0.25 m/km (16 inches/mi), but higher than that
obtained by the T-6600 profiler by 0.11 m/km (7 inches/mi) along
the right wheelpath. The T-6600 profiler conducted nine runs at
this site. Along the left wheelpath, the IRI for the nine runs
ranged from 1.80 to 2.00 m/km (114 to 127 inches/mi), while along
the right wheelpath, the IRI ranged from 1.70 to 1.86 m/km (108 to
118 inches/mi). This indicates there is some transverse variability
at this site. A comparison of the profile data from the T-6000 and
DNC 690 profilers indicated that there were localized differences
between the profile data and that these caused the difference in
the IRI from the profilers. The difference in IRI between the two
profilers has opposite signs for the two wheelpaths. This is
usually an indication that the difference in IRI between the two
profilers is probably related to variations in the profiled paths.
No explanation other than variability between the profiled paths
can be offered to explain the difference in IRI between the two
profilers at this site.

Site 4: IRI from the DNC 690 profiler at this
site was higher than that obtained by the T-6600 profiler by 0.16
m/km (10 inches/mi) and 0.12 m/km (8 inches/mi) along the left and
right wheelpaths, respectively. An evaluation of the roughness
profile for the left wheelpath indicated that most of the
difference in roughness between the two profilers was occurring
between 140 m (459 ft) and the end of the section. This was caused
by differences in the way a feature, which was located at
approximately 145 m (476 ft), was being measured by the two
profilers. An evaluation of the roughness profiles for the right
wheelpath also showed some localized variations in roughness that
occurred because of differences in the way features were measured
by the two profilers. Because this site is fairly rough, with left-
and right-wheelpath IRI values of approximately 2.10 and 2.14 m/km
(133 and 152 inches/mi), respectively, variations in the profiled
paths are probably the cause of the difference in IRI between the
two profilers.

Western Region

Figure 31 shows the difference in IRI from the DNC 690 and
T-6600 profilers at the sections tested by the two Western
profilers as a function of the IRI for the wheelpath. The
difference in IRI between the two profilers was within ±0.10 m/km
(±6 inches/mi) for all of the cases except one. This case was along
the left wheelpath at section 2, where the difference in IRI was
1.1 m/km (70 inches/mi). This occurred because data collected by
the DNC 690 profiler was contaminated by saturation spikes. This
data point is not shown in figure 31.

Cross Correlation of IRI

The cross-correlation technique provides a method to compare the
magnitude and the spatial distribution of IRI between two devices.
This technique was used to compare IRI obtained by the DNC 690 and
T-6600 profilers using data obtained during the 1996 verification
testing in the North Central and Western regions. When using the
cross-correlation technique, the DNC 690 profiler was considered to
be the "correct" device and, thus, the analysis will indicate how
well the T-6600 profiler reproduced the results from the DNC 690
profiler.

1 m/km = 5.28 ft/mi

Figure 31. Differences in IRI between the
K.J. Law Engineers DNC 690 and T-6600 profilers: Western
region.

One representative run was selected for each profiler at each
site to perform this analysis. In this analysis, for the T-6600
profiler, the ProQual-processed averaged data that are at 150-mm
(5.9-inch) intervals was used, while for the DNC 690 profiler, the
152.4-mm (6-inch) data obtained by the profiler was used. Because
the data collected by the DNC 690 profiler is considered to be the
correct data, any deviations in the path followed by the T-6600
profiler from the path followed by the DNC 690 profiler will affect
the results. The results of the crosscorrelation analysis are
presented in table 5.

Table 5. Results of cross correlation between the K.J. Law
Engineers DNC 690 and T-6600 profilers.

Region

Site

IRI (m/km)

Cross Correlation

Left Wheelpath

Right Wheelpath

Left Wheelpath

Right Wheelpath

DNC 690

T-6600

DNC 690

T-6600

North Central

1

1.00

1.04

1.06

1.02

0.79

0.91

North Central

2

3.87

4.10

4.96

4.81

0.81

0.94

North Central

3

1.06

1.07

1.03

1.07

0.85

0.95

North Central

4

2.65

2.64

2.90

2.92

0.92

0.96

Western

1

0.75

0.79

0.88

0.91

0.88

0.94

Western

2

3.37

2.51

2.62

2.62

0.13

0.85

Western

3

0.98

0.99

0.94

0.88

0.74

0.74

Western

4

2.24

2.32

2.40

2.45

0.88

0.93

1 m/km = 5.28 ft/mi

The sensor spacing for the two North Central profilers was
different. During testing, the two profilers aligned the right
sensor along a similar path using a camera system. The two North
Central profilers had very high cross-correlation values along the
right wheelpath where the camera system was used to judge the
wheelpath.

This indicates excellent agreement in both the IRI magnitude and
IRI distribution along that path for the two profilers. The two
North Central profilers also showed good cross-correlation values
along the left wheelpath, too, although the values were slightly
less than those obtained for the right wheelpath.

The two profilers in the Western region also had high
cross-correlation values at the majority of the sections. The data
collected along the left wheelpath at site 2 by the DNC 690
profiler were contaminated with saturation spikes, thus, a low
cross-correlation value was obtained for this case. The
cross-correlation values at site 3 were somewhat lower than the
values obtained for the other sites. Site 3 is a concrete site, and
evaluation of the profile data indicated that the amount of slab
curling that was present when the two profilers measured the site
was different, and this was the cause of the low cross correlation
at this site.

Analysis of Variance and Regression
Analysis of IRI

An analysis of variance (ANOVA) was performed using the IRI
values obtained from the 1996 regional testing to determine whether
IRI values obtained by the DNC 690 and T-6600 profilers were
similar. A two-factor ANOVA was performed using the IRI values
obtained for the five runs that were selected for computing the
average IRI values shown in table 4. During the 1996 verification
test, testing was performed at 16 sections (4 sections per region),
and this provided 32 cases (32 wheelpaths) that could be used in
the analysis. Because the data for the left wheelpath at site 2 in
the Western region for the DNC 690 profiler were erroneous, these
data were omitted from the analysis. The ANOVA indicated that the
profilers were significant at a significance level of 0.05.

Another ANOVA was performed by omitting the data for the left
wheelpath in the North Central region. This was done because the
two profilers in the North Central region have different sensor
spacing, and they were aligned along the right wheelpath during
testing. This analysis also found that the profilers were
significant at a significance level of 0.05. Thereafter, separate
ANOVAs were performed for each region. The only case where the
profilers were not significant was in the Western region.

Thereafter, for each region, separate ANOVAs were performed for
each wheelpath. The only cases where the profilers were not
significant at a significance level of 0.05 were for the left and
right wheelpaths in the Western region, and for the left wheelpath
in the North Central region.

A regression analysis was performed for the IRI from the DNC 690
and T-6600 profilers. The IRI for the five runs that were selected
at each section to compute the average IRI value in the previous
analysis were used in the regression. This provided 155 pairs of
data for the regression (i.e., four regions x four sections per
region x two wheelpaths per section x five runs per section, less
the erroneous left-wheelpath runs at section 2 in the Western
region). The following relationship was obtained from the
regression:

IRI (T-6600) = 0.982 IRI (DNC 690) + 0.004
(1)

where:

IRI (T-6600) = IRI from T-6600 profiler for a wheelpath
(m/km).

IRI (DNC 690) = IRI from DNC 690 profiler for a wheelpath
(m/km).

R2 = 0.99, standard error = 0.09 m/km.

The regression analysis indicated that IRI from the two
profilers were extremely similar; however, IRI obtained from the
DNC 690 profiler was predicted to be slightly higher than that
obtained from the T-6600 profiler.

COMPARISON OF K.J. LAW
ENGINEERS T-6600 AND ICC PROFILERS

Comparison of Profile
Data

The data obtained from the 2002 verification test were used for
this analysis. Both profilers applied a 100-m (328-ft)
upper-wavelength cutoff filter onto the data. The data collected by
both profilers along each wheelpath at each site were overlaid to
evaluate differences in the data. At some sites, the profiles
overlaid extremely well; however, at many sites, there were
significant differences among the profiles. Figure 32 shows an
example of a case where close agreement was obtained between the
profile data from the two profilers. Figure 33 shows an example of
a case where there were significant differences between the profile
plots. The data shown in figures 32 and 33 are the 25-mm (1-inch)
left-wheelpath data collected by the two Western profilers during
the 2002 verification test at LTPP sites 320209 and 069107,
respectively.

25.4 mm = 1 inch
1 m = 3.28 ft

Figure 32. Comparison of ICC and K.J. Law
Engineers profiles: Western site 320209.

An evaluation of all data collected for the 2002 verification
test indicated that the profile plots from the two profilers
usually overlaid well at sites that did not have much
long-wavelength content. However, differences between the profile
plots were noticeable at sites that had more long-wavelength
content. An evaluation of the profile data using filtering
techniques indicated that profile features that were present on the
pavement were being measured similarly by both profilers. These
observations indicate that there are differences in the
long-wavelength data collected by the two profilers. The
differences in the long wavelengths appear to be occurring for
wavelengths greater than approximately 40 m (131 ft).

25.4 mm = 1 inch
1 m = 3.28 ft

Figure 33. Comparison of ICC and K.J. Law
Engineers profiles: Western site 069107.

The response of the quarter car filter that is used in the IRI
computation procedure to different wavelengths is shown in figure
34.(2) The amplitude of the output sinusoid is the
amplitude of the input multiplied by the gain shown in the figure,
which is dimensionless. IRI is primarily influenced by wavelengths
ranging from 1.2 to 30.5 m (4 to 100 ft).(2) However,
there is still some response to wavelengths outside this range. The
IRI filter has a maximum sensitivity to sinusoids with wavelengths
of 2.4 and 15.4 m (7.9 and 50.5 ft). The response is down to 0.5
for wavelengths of 1.2 and 30.5 m (4 and 100 ft).(2)

1 m = 3.28 ft

Figure 34. Response of the IRI
filter.(2)

IRI obtained by the T-6600 and ICC profilers during the 2002
verification test showed good agreement.(28) Although
there are differences in the long wavelengths between the two
profilers, good agreement in IRI between the two profilers
indicates that the profilers are collecting similar data within the
wavelength range that is influencing the IRI that was described
previously.

The K.J. Law Engineers profiler uses a Butterworth filter for
long-wavelength cutoff, while the ICC profiler uses a cotangent
filter. Although both profilers are using an upper-wavelength
cutoff filter of 100 m (328 ft), differences in the filtering
techniques used by the two profilers are causing some differences
in the long-wavelength data between the two profilers. An
evaluation of the filtering techniques indicated that the
Butterworth filter makes a much sharper transition from wavelengths
that are unmodified to wavelengths that are eliminated than the
cotangent filter, and this causes differences in the long
wavelengths to occur between the two profilers.

Figure 35 shows a typical PSD plot that was obtained when the
25-mm (1-inch) data from the ICC and K.J. Law Engineers profilers
were compared. The data shown in figure 35 are those obtained by
the two North Central profilers at site 5 (which has a chip seal)
during the 2002 verification test. The two PSD plots show good
agreement between wave numbers of 0.025 and 1 cycle/m (0.008 and
0.305 cycle/ft), which correspond to wavelengths between 40 and 1 m
(131 and 3 ft). However, there are differences between the
profilers for wave numbers less than 0.025 cycle/m (0.008
cycle/ft), which corresponds to wavelengths greater than 40 m (131
ft), and wave numbers greater than 1 cycle/m (0.3 cycle/ft), which
corresponds to (wavelengths less than 1 m (3 ft).

1 cycle/m = 0.3 cycle/ft
1 m/cycle = 3 ft/cycle

Figure 35. PSD plot of 25-mm (1-inch)
data collected by the North Central ICC and K.J. Law Engineers
profilers at the chip-seal section during the 2002 verification
test.

An examination of PSD plots of data collected by the ICC and
K.J. Law Engineers profilers during the 2002 regional comparison
indicated the following:

The ICC and T-6600 profilers generally are collecting similar
data between wavelengths of 1 and 40 m (3 and 131 ft).

There were some differences in the data collected by the two
profilers for wavelengths greater than 40 m (131 ft). The PSD plots
indicated that, usually, the K.J. Law Engineers profiler was
collecting more spectral content than the ICC profiler for
wavelengths greater than 40 m (131 ft).

For wavelengths below 1 m (3 ft), the ICC profilers usually
have slightly more content compared to the K.J. Law Engineers
profiler. This may be occurring because of the small footprint size
of the height sensor on the ICC profiler. The laser height sensor
on the ICC profiler has a 1.5-mm- (0.06-inch-) diameter circular
footprint, while the K.J. Law Engineers profiler has an elliptical
footprint that is 38 by 6 mm (1.5 by 0.24 inches). The sensors in
the K.J. Law Engineers profiler are reported to be averaging the
elevation values within its footprint when obtaining height
measurements. This can cause elevation values obtained by the K.J.
Law Engineers profiler to be less than those obtained by the ICC
profiler in pavements that have coarse texture and in pavements
that have narrow upward or downward features.

A closeup view of the 25-mm (1-inch) profile data collected by
the two profilers at the site (whose PSD plot is shown figure 35)
is shown in figure 36. Figure 36 indicates that the profile
recorded by the ICC profiler shows more profile details compared to
those recorded by the K.J. Law Engineers profiler. This is why the
PSD plots shown in figure 35 indicate a difference between the two
profilers for wave numbers greater than 1 cycle/m (0.3
cycle/ft).

1 inch = 25.4 mm
1 m = 3.28 ft

Figure 36. Closeup view of 25-mm (1-inch)
profile data collected by North Central ICC and K.J. Law Engineers
profilers on a chip-seal pavement.

When ProQual processes the 25-mm (1-inch) data by applying a
300-mm (11.8-inch) moving average, the short-wavelength features
that have wavelengths less than 1 m (3 ft) will become attenuated
and the PSD plots for the two profilers will show better agreement
for wave numbers greater than 1 cycle/m (0.3 cycle/ft).

An analysis was performed to investigate the differences between
the two profilers in measuring short wavelengths. The data
collected by the North Central profilers during the 2002 regional
verification test were used for this analysis. At each site, one
run from each profiler was selected, and the analysis was performed
on the left-wheelpath data. The analysis was performed on the first
76 m (249 ft) of the data from the site. The profile data were
filtered to get rid of the long wavelengths, so that only the short
wavelengths would be present in the profile. Thereafter, the
standard deviations of the filtered elevation values were computed.
The results from this analysis are presented in table 6.

Table 6. Standard deviations of filtered elevation
values.

Site

Description

Standard Deviations of
Filtered Elevation (mm)

ICC

K.J. Law

1

Smooth Asphalt

0.0066

0.0064

2

Rough Asphalt

0.0174

0.0079

3

Smooth Concrete

0.0092

0.0082

4

Rough Concrete

0.0115

0.0084

5

Chip Seal

0.0113

0.0100

1 inch = 25.4 mm

The ICC profiler had higher standard deviations for all of the
cases. The data shown in table 6 suggests that the small footprint
size of the ICC profiler is measuring more texture-related effects
and possibly higher depth at narrow downward features, such as
cracks, than that measured by the T-6600 profiler.

Comparison of 25-mm (1-inch) data collected by the K.J. Law
Engineers and ICC profilers indicated that there were differences
in the depth of downward features, such as cracks, that were
measured by the profilers at some sites. However, it is unclear if
this difference was caused by variations in the paths profiled by
the two profilers, or if it was related to differences in the
footprint sizes of the two height sensors, or differences in the
filtering techniques used by the two profilers.

Figure 37 shows a closeup view of the profile data recorded by
the two profilers over a joint in a concrete pavement. The data
shown in this figure were collected by the two North Central
profilers at the rough PCC section during the 2002 verification
test. On this pavement, minor variations in the profiled path are
not likely to result in a difference in the magnitude of the
downward feature measured by the profilers when readings are taken
on top of the sealant at a joint. This is because the joint sealant
is expected to be at a constant depth from the slab surface over a
short lateral distance. However, in cases where there are
transverse cracks in the pavement, minor variations in the
wheelpaths can result in the crack depth being different. Thus,
comparing the magnitude of the downward feature recorded by the two
profilers at a joint provides a better choice for comparing the two
profilers. The two plots in figure 37 show that the depths of the
joint as recorded by the two profilers are very similar; however,
the depth recorded by the ICC profiler is slightly higher.

25.4 mm = 1 inch
1 m = 3.28 28 ft

Figure 37. Readings taken over a joint by
the two profilers.

Figure 38 shows the 25-mm (1-inch) profile data collected by the
two profilers at this site after the data have been subjected to a
3-m (10-ft) high-pass filter. The slab length in this concrete
pavement is 13 m (43 ft). Figure 38 shows that both profile plots
are showing all joint locations, and the depths recorded over the
joints by the two profilers are very similar.

25.4 mm = 1 inch
1 m = 3.28 28 ft

Figure 38. Profile data obtained by the
ICC and K.J. Law Engineers profilers at a concrete
site.

Although the profilers output data at 25-mm (1-inch) intervals,
the height sensors in the profilers are obtaining data at much
closer intervals, and are then averaging the data and using the
averaged height-sensor value for computing the profile data at
25-mm (1-inch) intervals. If the profilers were just getting
height-sensor data at 25-mm (1-inch) intervals, there is always the
possibility that a reading may not be obtained on top of a joint.
However, because the height sensors are obtaining readings at much
closer intervals than 25 mm (1 inch), a reading is always obtained
on top of the joint sealant. When the height-sensor data are
averaged to obtain a reading every 25 mm (1 inch), the reading
obtained over the joint was of sufficient magnitude for the joint
to be clearly seen in the filtered profile.

Another interesting observation seen in figure 37 is that the
profile data show the joint to be a feature that is spread over a
distance of 75 mm (3 inches), when the actual width of the joint is
on the order of 10 mm (0.4 inches). The joint appears like this in
the profile data because of the averaging procedure that is used on
the height-sensor data and possibly because of the application of
an anti-aliasing filter onto the profile data. The averaging
procedure and the antialiasing filter will also cause some
attenuation in the magnitude of the depth of narrow downward
features such as joints and cracks.

Comparison of IRI
Values

An analysis of the IRI values obtained from the 2002
verification test was performed to compare IRI values obtained by
the two profilers.(28) In the North Central region,
testing by both profilers was performed on the same day. In the
North Atlantic region, testing at six of the eight sites was
performed on the same day by the two profilers, while in the
Western region, a similar procedure was followed for three of the
five sites. In the Southern region, testing at the sites by the ICC
profiler was performed approximately 1.5 months after testing by
the K.J. Law Engineers profiler. The IRI values obtained from the
testing (average IRI from five runs) and the test dates are
presented in appendix B.

There were 23 test sites (46 wheelpaths) where IRI comparisons
of the two profilers could be made. Figure 39 shows the IRI
relationship between the two profilers, where data for 46
wheelpaths are shown. There is very good agreement in IRI between
the two profilers, with the correlation coefficient for the two
sets of IRI values being 0.99.

1 m/km = 5.28 ft/mi

Figure 39. Relationship between IRI from
the K.J. Law Engineers and ICC profilers.

The difference in IRI between the K.J. Law Engineers and ICC
profilers (K.J. Law IRI – ICC IRI) was computed along each
wheelpath for all test sections. The differences in IRI are shown
in figure 40 as a function of the IRI for the wheelpath, where the
IRI for the wheelpath was computed by averaging IRI obtained for
that wheelpath by the ICC and K.J. Law Engineers profilers.

1 m/km = 5.28 ft/mi

Figure 40. Differences in IRI between the
K.J. Law Engineers and ICC profilers.

For the 46 cases, the difference in IRI was within ±0.10 m/km
(±6 inches/mi) for 33 cases, between 0.10 and 0.20 m/km (6 and 13
inches/mi) for 6 cases, between -0.10 and -0.20 m/km (-6 and -13
inches/mi) for 4 cases, between -0.20 and -0.30 m/km (-13 and -19
inches/mi) for 2 cases, and between 0.30 and 0.40 m/km (19 and 25
inches/mi) for 1 case. An investigation was performed separately
for each region to identify the cause of the difference in IRI
between the two profilers for cases where the difference in IRI was
outside ±0.10 m/km (±6 inches/mi).

North Atlantic Region

Figure 41 shows the difference in IRI between the K.J. Law
Engineers and ICC profilers at the sections tested by the North
Atlantic profilers as a function of the IRI for the wheelpath.

1 m/km = 5.28 ft/mi

Figure 41. Differences in IRI between the
K.J. Law Engineers and ICC profilers: North Atlantic
region.

Differences in IRI between the K.J. Law Engineers and ICC
profilers (K.J. Law IRI — ICC IRI) that were outside ±0.10 m/km (±6
inches/mi) were observed for the following four cases: (1) left
wheelpath of site 251002 (a difference of -0.23 m/km (-15
inches/mi)), (2) right wheelpath of site 251002 (a difference of
0.17 m/km (11 inches/mi)), (3) left wheelpath of site 364018 (a
difference of -0.23 m/km (-15 inches/mi)), and (4) left wheelpath
of site 245807 (a difference of 0.15 m/km (10 inches/mi)).

Site 251002: At this site, the K.J. Law
Engineers profiler obtained an IRI that was 0.23 m/km (15
inches/mi) lower than the IRI from the ICC profiler for the left
wheelpath, and an IRI that was 0.17 m/km (11 inches/mi) higher than
the IRI from the ICC profiler for the right wheelpath. Each
profiler conducted nine runs on this section. The IRI for the nine
runs from the ICC profiler ranged from 2.58 to 5.03 m/km (164 to
319 inches/mi) for the left wheelpath, and from 1.24 to 1.57 m/km
(79 to 100 inches/mi) for the right wheelpath. For the K.J. Law
Engineers profiler, the IRI for the nine runs ranged from 2.56 to
4.81 m/km (162 to 305 inches/mi) for the left wheelpath, and from
1.50 to 1.67 m/km (95 to 106 inches/mi) for the right wheelpath.
The left wheelpath of this section had significant distress. As
indicated from the IRI range that was obtained for the repeat runs,
significant variability in IRI can occur at this site because of
variability in the profiled path. Investigation of the profile data
indicated that the difference in IRI between the two profilers at
this site was probably caused by differences in the profiled
paths.

Site 364018: The K.J. Law Engineers profiler
obtained an IRI that was 0.23 m/km (15 inches/mi) less than that
obtained by the ICC profiler along the left wheelpath at this
section. Each profiler conducted nine runs on this test section.
The IRI for the runs for the ICC profiler ranged from 2.64 to 3.18
m/km (167 to 202 inches/mi) for the left wheelpath, and from 2.20
to 2.39 m/km (139 to 151 inches/mi) for the right wheelpath. For
the K.J. Law Engineers profiler, IRI ranged from 2.59 to 2.92 m/km
(164 to 185 inches/mi) for the left wheelpath, and from 2.23 to
2.36 m/km (141 to 149 inches/mi) for the right wheelpath. There was
a major downward feature at a distance of 50 m (164 ft) along the
left wheelpath that made a significant contribution to the
roughness at this site. Variability in the profiled path that
caused this feature to be measured differently had a significant
effect on IRI. Investigation of the profile data and roughness
profiles at this section indicated that the difference in IRI
between the two profilers was caused by variability in the paths
followed by the two profilers.

Site 245807: Along the left wheelpath at this
site, the IRI from the K.J. Law Engineers profiler was 0.15 m/km
(10 inches/mi) lower than that obtained by the ICC profiler. An
investigation of the profile data did not indicate a clear reason
for the cause of this difference.

North Central Region

Figure 42 shows the difference in IRI between the K.J. Law
Engineers and ICC profilers at the test sections tested by the
North Central profilers as a function of the IRI for the
wheelpath.

Differences in the IRI between the K.J. Law Engineers and ICC
profilers (K.J. Law IRI – ICC IRI) that were outside ±0.10 m/km (±6
inches/mi) were observed for the following three cases: (1) right
wheelpath of site 17A002 (a difference of 0.15 m/km (10
inches/mi)), (2) left wheelpath of site 17A005 (a difference of
0.35 m/km (22 inches/mi)), and (3) right wheelpath of site 17A005
(a difference of -0.13 m/km (-8 inches/mi)).

1 m/km = 5.28 ft/mi

Figure 42. Differences in IRI between the
K.J. Law Engineers and ICC profilers: North Central
region.

Site 17A002: The IRI from the K.J. Law
Engineers profiler was higher than that of the ICC profiler by 0.15
m/km (10 inches/mi) along the right wheelpath at this site.
However, along the left wheelpath, the IRI from the K.J. Law
Engineers profiler was 0.10 m/km (6 inches/mi) lower than that
obtained by the ICC profiler. An investigation of the profile data
did not indicate a definitive cause for the difference in IRI
between the profilers. However, variability in the wheelpath is a
likely cause for the difference in IRI. In this case, the
difference in IRI between the K.J. Law Engineers and ICC profilers
had opposite signs for the wheelpaths (negative for the left
wheelpath and positive for the right wheelpath). This is an
indication that the two profilers followed different
wheelpaths.

Site 17A005: The IRI from the K.J. Law
Engineers profiler was higher than that obtained by the ICC
profiler by 0.35 m/km (22 inches/mi) along the left wheelpath;
along the right wheelpath, the IRI from the K.J. Law Engineers
profiler was lower than that obtained by the ICC profiler by 0.13
m/km (8 inches/mi). An investigation of the profile data did not
indicate a definitive cause for the difference in IRI between the
profilers. Since there was a reversal in signs for the difference
in IRI for the two wheelpaths as in the previous case, the
differences in IRI between the two profilers at this site were
probably cased by variability in the paths followed by the two
profilers.

Southern Region

Figure 43 shows the difference in IRI between the K.J. Law
Engineers and ICC profilers at the sections tested by the Southern
profilers as a function of the IRI for the wheelpath.

Differences in the IRI between the K.J. Law Engineers and ICC
profilers (K.J. Law IRI — ICC IRI) that were outside ±0.10 m/km (±6
inches/mi) were observed for the following three cases: (1) left
wheelpath of site 48B350 (a difference of 0.17 m/km (11
inches/mi)), (2) right wheelpath of site 48B350 (a difference of
-0.13 m/km (-8 inches/mi)), and (3) right wheelpath of site 485253
(a difference of 0.11 m/km (7 inches/mi)). The data recorded by the
ICC profiler could not be converted to obtain the 25-mm (1-inch)
data, thus, a comparison of the profiles between the ICC and K.J.
Law Engineers profilers could not be performed.

1 m/km = 5.28 ft/mi

Figure 43. Differences in IRI between the
K.J. Law Engineers and ICC profilers: Southern region.

At site 48B350, the IRI from the K.J. Law Engineers profiler was
higher than that obtained by the ICC profiler by 0.17 m/km (11
inches/mi) along the left wheelpath; however, along the right
wheelpath, the IRI from the K.J. Law Engineers profiler was lower
than that obtained by the ICC profiler by 0.13 m/km (8 inches/mi).
Since the difference in the IRI between the two profilers had
opposite signs for the left and right wheelpaths, variability in
the paths followed by the two profilers is a likely cause of the
difference in the IRI between the two profilers.

Western Region

Figure 44 shows the difference in IRI between the K.J. Law
Engineers and ICC profilers at the sections tested by the Western
profilers as a function of the IRI for the wheelpath.

Differences in IRI between the K.J. Law Engineers and ICC
profilers (K.J. Law IRI — ICC IRI) that were outside ±0.10 m/km (±6
inches/mi) were observed for the following three cases: (1) left
wheelpath of site 320209 (a difference of -0.18 m/km (11
inches/mi)), (2) right wheelpath of site 320209 (a difference of
-0.15 m/km (–10 inches/mi)), and (3) right wheelpath of site 067454
(a difference of 0.13 m/km (8 inches/mi)).

Site 320209: The IRI from the K.J. Law
Engineers profiler was lower than that obtained by the ICC profiler
by 0.18 m/km (11 inches/mi) and 0.15 m/km (10 inches/mi) along the
left and right wheelpaths, respectively. The two profilers measured
this section on different dates. An evaluation of the profile data
indicated that the amount of slab curling present when the ICC
profiler profiled the section was slightly higher than the curling
that was present when the site was profiled by the K.J. Law
Engineers profiler. The higher IRI obtained by the ICC profiler is
attributed to higher slab curling.

1 m/km = 5.28 ft/mi

Figure 44. Differences in IRI between the
K.J. Law Engineers and ICC profilers: Western region.

Site 067454: The IRI from the K.J. Law
Engineers profiler was higher than that obtained by the ICC
profiler by 0.13 m/km (8 inches/mi) along the right wheelpath. Each
profiler conducted nine runs on this section. The right-wheelpath
IRI from the ICC profiler for these runs ranged from 2.17 to 2.39
m/km (138 to 152 inches/mi), while the IRI from the K.J. Law
Engineers profiler for the same wheelpath ranged from 2.31 to 2.48
m/km (146 to 157 inches/mi). As seen from these values, there is
some overlap in the IRI values obtained by the two profilers.
Analysis of the profile data did not indicate a clear reason why
the K.J. Law Engineers profiler IRI would be higher. Variability in
the profiled paths may be a reason why the IRI values were
different between the devices.

Cross Correlation of IRI

The 25-mm (1-inch) profile data collected by the two North
Central profilers during the 2002 regional verification test were
used in this analysis. The first profile run conducted by each
profiler at the test sites was selected for analysis. The T-6600
profiler was considered to be the "correct" device and, thus, the
analysis will indicate how well the ICC profiler reproduced the
results obtained by the T-6600 profiler. No markings were present
at the test sections, and the driver of each profiler visually
judged the location of the wheelpaths when profiling the test
sections. Any variations in the profiled paths between the two
profilers will result in lower crosscorrelation values. The results
from the cross-correlation analysis are presented in table 7.

Good cross-correlation values were obtained for the T-6600 and
ICC profilers, which indicate good agreement in IRI magnitude and
IRI distribution between the two profilers. It should be noted that
other combinations of runs used for cross-correlation analysis
could give different results.

Table 7. Results of cross correlation between the K.J. Law
Engineers T-6600 and ICC profilers.

Site

IRI (m/km)

Cross Correlation

Left Wheelpath

Right Wheelpath

Left Wheelpath

Right Wheelpath

K.J. Law

ICC

K.J. Law

ICC

1: Asphalt

1.05

0.96

1.19

1.20

0.81

0.94

2: Asphalt

2.71

2.86

2.87

2.72

0.91

0.91

3: Concrete

1.11

1.10

1.18

1.19

0.82

0.80

4: Concrete

4.02

4.11

4.14

4.19

0.91

0.93

5: Chip Seal

3.39

3.01

3.85

4.00

0.78

0.85

1 m/km = 5.28 ft/mi

ANOVA and Regression Analysis of
IRI

An ANOVA was performed using the IRI data obtained from the 2002
regional verification test. There were 23 sites used for testing,
and this provided data for 46 cases (23 sites x 2 wheelpaths per
site) where an IRI comparison could be performed for the K.J. Law
Engineers and ICC profilers. For each case, IRI values for five
repeat runs were available for both the ICC and K.J. Law Engineers
profilers. The ANOVA indicated that the profilers were not
significant at a significance level of 0.05.

A regression analysis was performed for the IRI from the T-6600
and ICC profilers. The IRI for the five runs that were selected at
each section to compute the average IRI value for the IRI
comparison of the two profilers were used in the regression. This
provided 230 pairs of data for the regression (i.e., 23 sections x
2 wheelpaths x 5 repeat runs). The following relationship was
obtained from the regression:

The regression equation indicated very good agreement in IRI
between the two profilers.

EFFECTS OF APPLYING A MOVING AVERAGE
ONTO PROFILE DATA

The DNC 690 profilers collected profile data at 25.4-mm (1-inch)
intervals and then applied a 304.8-mm (12-inch) moving average onto
the data and recorded profile data at 152.4-mm (6- inch) intervals.
Profile data collected at 25-mm (1-inch) intervals at LTPP test
sections are available for both the T-6600 and ICC profilers.
However, currently, in the LTPP program, these 25-mm (1-inch)
profile data are processed using the ProQual software, which
applies a 300-mm (11.8-inch) moving average onto the 25-mm (1-inch)
profile data, and extracts profile data at 150-mm (5.9-inch)
intervals. The IRI values for the LTPP sections are computed using
this averaged data, and the averaged data are uploaded to the LTPP
database.

The application of the 300-mm (11.8-inch) moving average onto
the 25-mm (1-inch) data attenuates very short-wavelength features
that are present in the profile, and can also distort profile
features that are actually present in the pavement. The averaged
profile can show features that do not actually appear in the
pavement, while not showing features that are actually present in
the pavement. In this section, several examples that show the
distortion caused in profile features by the application of the
moving average are presented.

Faulted Pavement

Figure 45 shows how the application of the moving average
distorts the profile data collected over a fault. This figure shows
the 25-mm (1-inch) interval data collected by the North Central
T-6600 profiler over a faulted crack at the rough PCC section
during the 1996 profiler verification study. The averaged data also
are shown in this figure. The 25-mm (1-inch) data clearly define
the fault, which is about 13 mm (0.5 inches). However, the
application of the moving average makes the fault appear as a ramp,
where there is a gradual change in elevation of about 7 mm (0.3
inches) that occurs over a distance of 0.3 m (1 ft). As seen in
this example, the application of the moving average distorts the
profile and shows a feature that does not actually exist on the
pavement.

25.4 mm = 1 inch
1 m = 3.28 ft

Figure 45. Profile distortion caused by
the application of a moving average onto data collected over a
fault.

Effects of Downward Features

The rough AC section that was used in the 2003 LTPP profiler
comparison conducted in Minnesota had cracks that had been patched
full width across the lane. Figure 46 shows the profile data
obtained by the North Atlantic ICC profiler over such a patched
crack. The figure shows the 25-mm (1-inch) data and the averaged
data after the 25-mm (1-inch) data were processed using ProQual.
The 25-mm (1-inch) data indicate that the patched crack is about 9
mm (0.4 inches) deeper than the adjacent pavement area. The
application of the moving average onto the 25-mm (1-inch) data
causes the depth of the patched crack to be reduced.

25.4 mm = 1 inch
1 m = 3.28 ft

Figure 46. Profile distortion caused by
the application of a moving average onto data collected over a
patched crack.

Figure 47 shows the 25-mm (1-inch) data and the averaged 150-mm
(5.9-inch) interval data at the chip-seal section that was used in
the 2003 LTPP profiler comparison. These data were collected by the
North Atlantic ICC profiler. The 25-mm (1-inch) data show a crack
that is at a distance of about 0.9 m (3 ft) as a sharp and narrow
downward feature. However, the averaged data distorts the shape of
the crack and makes the crack appear as a dip that is spread over a
much wider length. The small variations between the profile data
points that are seen in the 25-mm (1-inch) data are not seen in the
averaged data. These variations are smoothed out by the application
of the moving average.

25.4 mm = 1 inch
1 m = 3.28 ft

Figure 47. Profile distortion caused by
the application of a moving average onto data collected over a
crack.

Figure 48 shows profile data collected by the Western ICC
profiler at site 3, which is a concrete section, during the 2003
profiler comparison in Minnesota. This figure shows both the 25-mm
(1-inch) data and the averaged data after the 25-mm (1-inch) data
were processed using ProQual. The 25-mm (1-inch) data clearly show
the locations of the joints in the concrete pavement as downward
spikes that occur at regular intervals. However, these features are
not seen in the averaged profile because the averaging process
attenuates the sharp downward features seen at the joints.

25.4 mm = 1 inch
1 m = 3.28 ft

Figure 48. Application of a moving
average onto data collected for a concrete pavement.

Effects of Sharp Upward
Features

Figure 49 shows a portion of the profile data collected by the
North Central T-6600 profiler at section 3, which is a PCC section,
during the 1996 regional verification test. The figure shows the
25-mm (1-inch) data and the averaged data after ProQual had
processed the 25-mm (1-inch) data. The profile contains a sharp
upward feature about 2.5 mm (0.1 inch) in height near 62 m (203
ft). The application of the moving average eliminates this feature.
The application of a moving average onto a sharp upward feature
that has a greater magnitude than the shown feature will cause the
feature to appear in the averaged data as a feature that has a much
lesser magnitude that is spread out over a much greater distance
than the actual feature. The profile shown in figure 49 also shows
a narrow downward feature between 65 and 66 m (213 and 216 ft).
This feature also does not appear in the averaged data.

Smooth Asphalt Pavement

Figure 50 shows a plot that contains profile data at 25-mm
(1-inch) intervals, and the same data after it had been processed
using ProQual. The profile shown in figure 50 contains data
collected by the Western profiler along the left wheelpath at the
smooth AC site during the 2003 LTPP profiler comparison. This
pavement section is a smooth pavement, and there is no distress
within the limits of the profile plot shown in figure 50. Both the
25-mm (1-inch) data and the averaged 150-mm (5.9-inch) data overlay
well, except that the small spike seen at 16 m (52 ft) does not
appear in the averaged data.

25.4 mm = 1 inch
1 m = 3.28 ft

Figure 49. Application of a moving
average onto a profile containing a sharp upward
feature.

SUMMARY OF THE FINDINGS

Data recorded by inertial profilers do not accurately portray
very narrow features such as cracks or joints in PCC pavements
because of the low-pass filtering that is performed on the data.
Evaluation of 25-mm (1-inch) data collected by both the T-6600 and
ICC profilers over a joint in a PCC pavement showed that the joint
appeared in the profile as a feature that was spread over a
distance of 75 mm (3 inches), when the width of the joint was
actually closer to 10 mm (0.4 inches). Although 25-mm (1-inch)
interval data are collected by both the T-6600 and ICC profilers,
the height sensors in the profilers collect data at much closer
intervals and then average the data when computing profile data at
25-mm (1-inch) intervals. This causes the magnitude of a narrow
feature that is recorded in the profile to be less than the actual
magnitude, and also causes it to be spread out over a much wider
distance than the actual feature. The application of an
anti-aliasing filter onto the profile data can also have the same
effect.

Since the DNC 690 profiler recorded profile data at 152.4-mm
(6-inch) intervals, when comparing data from the T-6600 profiler
with the DNC 690 profiler, only the 150-mm (5.9-inch) interval
ProQual-processed data from the T-6600 profiler can be used to
perform a meaningful comparison. Comparison of the profile plots
for the two profilers showed good agreement, although there were
some differences between the profiles for sections that had
significant longwavelength content. This difference is attributed
to the different long-wavelength cutoff filter values used for the
two profilers (91 m (300 ft) for the DNC 690 profiler and 100 m
(328 ft) for the T-6600 profiler). An evaluation of the profile
data indicated that the long-wavelength cutoff filtering technique
used in both the DNC 690 and T-6600 profilers appears to be
similar. There was very good agreement in IRI values for the DNC
690 and T-6600 profilers.

Since 25-mm (1-inch) interval data were available for both the
T-6600 and ICC profilers, a comparison of 25-mm (1-inch) interval
data for the two profilers could be performed. Evaluation of the
profile data using PSD plots indicated that there was good
agreement in the profile data for the two profilers for wavelengths
between 1 and 40 m (3 and 131 ft). For wavelengths less than 1 m (3
ft), the ICC profiler usually showed a higher wavelength content
than the T-6600 profiler. This is attributed to the smaller
footprint of the ICC profiler, which probably caused more texture
effects and the higher magnitude of narrow features to be recorded.
For wavelengths greater than 40 m (131 ft), the T-6600 profiler
recorded more wavelength content than the ICC profiler. This is
attributed to differences in the long-wavelength filtering
techniques that are used by the two profilers. When
ProQual-processed data for the two profilers were compared using a
PSD plot, only the differences at the higher wavelengths will be
seen, with good agreement being obtained between the two profilers
for wavelengths less than 1 m (3 ft). This occurs because the
application of the moving average attenuates the short-wavelength
features. Good agreement in IRI, which is primarily influenced by
wavelengths between 1 and 30 m (3 and 100 ft), was obtained for
data collected by the ICC and T-6600 profilers.

In the LTPP program, the 25-mm (1-inch) data obtained from the
T-6600 and ICC profilers are processed using ProQual. ProQual
applies a 300-mm (11.8-inch) moving average onto the 25-mm (1-inch)
data and extracts data at 150-mm (5.9-inch) intervals, and these
data are uploaded to the LTPP database. The application of the
moving average onto the 25-mm (1-inch) data attenuates features
with wavelengths less than 1 m (3 ft). Detailed profile features
cannot be observed in the ProQual-processed data because of this
effect. The moving average can also distort the profile data, with
the averaged data showing features that are not actually present in
the pavement, while eliminating features that are actually
present.